package/tftpd: add an upstream URL to Config.in

Add an upstream URL to the help text in Config.in. This
addresses the 'Missing' URL status in the package stats
web page output.

Signed-off-by: Mark Corbin <mark@dibsco.co.uk>
[yann.morin.1998@free.fr:
  - use the git tree instead of the 8-year old freshmeat webpage
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/tftpd/Config.in

@@ -6,3 +6,5 @@ config BR2_PACKAGE_TFTPD
 	depends on BR2_PACKAGE_BUSYBOX_SHOW_OTHERS
 	help
 	  HPA's Trivial File Transfer Protocol (tftp) server.
+
+	  https://git.kernel.org/pub/scm/network/tftp/tftp-hpa.git